Of the reported phases of iron oxide, only hematite (α-Fe2O3) is considered a promising anode material for photoelectrochemical (PEC) oxidation of water for application in solar fuel production. Indeed, α-Fe2O3 possesses a suitable band gap and alignment for solar absorption and the O2 evolution reaction of water, is terrestrially abundant and inexpensive, and is stability in aqueous, and is therefore considered one of the most attractive options. Within the last 10 years, the improved quality and nanoscale control of α-Fe2O3 photoanodes has spurred a series of record-breaking efficiencies. However, the major limiting factors of hematite is the high recombination rate of photo-generated charge carriers combined with the relatively weak absorption of light, as well as slow water oxidation kinetics at the α-Fe2O3 surface. This means less than 50% of visible light is absorbed within the space-charge layer and can contribute to PEC water splitting. While efforts have been made to improve the intrinsic charge-transport and oxidation kinetic properties of α-Fe2O3 by means of doping and surface modification, α-Fe2O3 may be limited by these properties. While it is difficult to find the same beneficial combination of photoanode properties in other semiconducting materials, it is possible that alternate iron oxide polymorphs may exhibit superior behavior.
Phoelectrochemical investigations of the other reported iron oxide phases, which include Fe1-xO, β-Fe2O3, γ-Fe2O3, ε-Fe2O3, and Fe3O4, have been largely overlooked in favor of research on α-Fe2O3. This stems from the fact that these polytypes are either metastable and difficult to synthesize in useful forms (β-Fe2O3 and ε-Fe2O3), or have not demonstrated significant photoactivity (Fe1-xO, γ-Fe2O3, Fe3O4). The β-Fe2O3 polymorph, which is our focus here, has no reported natural occurrence and as of yet has found little application. It has been synthesized as a polycrystalline thin film on only a handful of occasions by using iron trifluoroacetylacetone as an iron source in chemical vapor deposition and spray pyrolysis. More commonly, it has been produced as nanoparticulate form via hydrolysis of FeCl3, solid-state reaction of NaCl and Fe2(SO4)3, or via thermal decomposition of Fe(C10H9CHO) or FeSO4 in mesoporous SiO2. While monophasic nanoparticles of β-Fe2O3 have been reported, it appears that all β-Fe2O3 thin films fabricated to date have been mixed-phase.
β-Fe2O3 possesses a bixbyite-type crystal structure (space group Ia3) with lattice parameter a=9.40. β-Fe2O3 is thermodynamically unstable, with reports of transformation to either α-Fe2O3 or γ-Fe2O3 upon heating, depending on the morphology and annealing conditions. While pure hematite is a canted antiferromagnet or weak ferromagnet at room temperature, β-Fe2O3 is paramagnetic at room temperature.
There is therefore a substantial need for an improved article of manufacture to provide highly efficient solar energy conversion for photochemical oxidation of water and which can be efficiently and rapidly manufactured.